This value is determined experimentally and represents the enthalpy change when one mole of isobutane is formed from its constituent elements in their standard states under standard conditions (298 K and 1 atm).
Here's the balanced chemical equation for the standard enthalpy change of formation of isobutane:
4 C(graphite) + 5 H2(g) → C4H10(g) ΔHf° = -134.5 kJ/mol
This equation shows that 4 moles of solid carbon (graphite) and 5 moles of hydrogen gas react to form 1 mole of gaseous isobutane, releasing 134.5 kJ of heat in the process.
